Below is a brief selection of sources for statistics and numbers. If you are having trouble, finding appropriate numbers, ask your professor and/or a librarian. If they don't know where the data exists, they might now how to look for it.
Articles often contain sourced statistics, so it is good to read carefully and make note of where any mentioned statistical data comes from.
As a general rule, start with the Pew Research Center for public opinion information gathering on major subjects. For government data, go to the appropriate agency or try searching USA.gov, which is the government landing page for agencies, studies, statistics, and data. States all have some form of public-facing data, the same goes for large U.S. cities.